As is known, radio frequency (RF) transmitters modulate baseband signals, such as analog voice or digital voice samples, onto an RF carrier, amplify the RF carrier, and transmit the RF carrier, via an antenna, through the air as electromagnetic energy. The electromagnetic energy is subsequently received by a receiver's antenna, demodulated back to the baseband signal, and rendered audible (if voice was transmitted) by the receiver.
As is also known, many communication systems, such as cellular telephone and trunking, utilize spectrally efficient modulation techniques, such as quadrature amplitude modulation (QAM) and quatemary phase shift keying (QPSK) in a time division multiple access (TDMA) format. These spectrally efficient modulation techniques typically correlate the baseband signal to changes in RF carrier amplitude and phase via a digital symbol constellation format such as the QAM format, which is described in co-pending U.S. patent application Ser. No. 07/783,289, entitled "Communication Signal Having A Time Domain Pilot Component", assigned to Motorola Inc. Since the spectrally efficient modulation techniques require variation of the RF carrier amplitude, a linear class A or class AB amplifier must be used. If the amplifier is not linear, it provides unwanted RF energy, or splatter, at frequencies adjacent to the RF carrier. This splatter may subsequently interfere with two-way communications in process on the adjacent frequencies, or channels.
Linearity of an amplifier is affected by the varying loads presented by the antenna of the transmitter. Typically, an antenna is designed to provide a fixed load, fifty ohms for example. However, due to reflected energy received, the load changes.
To minimize variations in loading, transmitters generally incorporate isolators to provide a substantially constant load impedance to the amplifier. The isolator includes a circulator and a terminating impedance which is typically fifty ohms. The circulator is a three terminal device that provides directional flow of the RF energy from the amplifier to the antenna and from the antenna to the terminating impedance. Therefore, the RF energy sourced by the amplifier is provided to the antenna and any RF energy entering the antenna is absorbed in the terminating impedance. Thus, the isolator presents a constant impedance to the amplifier.
Although the isolator provides a constant load impedance to the RF amplifier, size, cost, and bandwidth limitations typically inhibit the use of a universal isolator in mobile radios, portable radios, and cellular telephones. For example, a radio that operates at 132 MHz requires an isolator that has a volume of 8.19 cubic centimeters (0.5 cubic inches), weighs 227 grams (0.5 pounds), and costs $30 each in quantities of 100,000 per year. As a result, an isolator puts obvious constraints on the design of such radios. Additionally, isolators have fixed bandwidths; therefore, multiple isolators may be required in transmitters which operate over wide frequency ranges. This bandwidth limitation is most noticeable at lower RF carrier frequencies, such as VHF, where the allocated frequency band covers a large percentage bandwidth.
To avoid the use of the isolator, existing frequency modulation (FM) transmitters, which include nonlinear amplifiers, utilize protective feedback circuitry. The protective feedback monitors the voltage standing wave ratio (VSWR) at the nonlinear amplifier's output and correspondingly reduces the amount of output power provided by the nonlinear amplifier to the antenna. This approach generally reduces the nonlinear amplifier's output power by a fixed amount when the VSWR exceeds a predetermined level. For example, when a 3:1 VSWR is detected at the nonlinear amplifier's output, the nonlinear amplifier's output power may be reduced by 3 dB. This approach works for nonlinear amplifiers, but does not include any provisions for maintaining amplifier linearity.
Therefore, a need exists for a method to compensate for effects of varying loading without the need for an isolator in a transmitter that has a linear amplifier.